Such a reference message for generating basic cycles as part of a time-triggered communication on a bus system is used in FlexRay or in TTCAN, for example. Such bus systems are necessary because the interconnection of control units, sensors, and actuators has recently increased drastically in the production of modern vehicles and in mechanical engineering, in particular in the machine tool area, as well as in automation. Synergistic effects due to distributing functions among multiple control units may be achieved in this way. Communication among various stations in such distributed systems takes place increasingly via a bus system. Communications traffic on the bus system, access and reception mechanisms, and fault handling are regulated via a protocol.
The protocol, which is established in the automotive field, for example, is a CAN (Controller Area Network) which is an event-controlled protocol, i.e., protocol activities such as the sending of a message are initiated by events having their origin outside of the communication system, i.e., the bus system. This is thus an event-controlled bus system which may be triggered in particular by external events. In this way the CAN protocol is very flexible and adding additional users and messages is thus possible with no problem.
An alternative approach for event-controlled spontaneous communication is the purely time-triggered approach. All communication activities on the bus are strictly periodic. Protocol activities such as sending a message are triggered only by the progression of a time which is valid for the entire bus system. Access to the medium is based on the allocation of time ranges in which a transmitter has an exclusive transmission right. The protocol is comparatively inflexible, and new nodes are only able to be added when the corresponding time ranges have been left free. This circumstance requires the message sequences to be set before the starting operation. Thus a timetable is created that must comply with the message requirements regarding repeat rate, redundancy, deadlines, etc. Positioning of the messages within the transmission periods must be coordinated with the applications which produce the message content to minimize latencies between application and sending instant. Without this coordination, the advantage of time-control transmission, i.e., minimal latency and jitter when sending the message on the bus, would be lost. High demands are thus placed on the planning tools in a purely time-triggered approach.
The method of achieving this object of a time-triggered controller area network known as TTCAN (Time-Triggered Controller Area Network) as defined in the ISO 11898-4 Standard Draft from 2003 (ISO/TC 22/SC3) meets the requirements outlined above for time-triggered communication and the requirements for a certain degree of flexibility. TTCAN fulfills these requirements by constructing communication cycles, known as basic cycles, by sending a reference message through the timer, known as the time master. These basic cycles are then divided into exclusive time windows for periodic messages of certain communication users and into arbitrating time windows for spontaneous messages of multiple communication users.
The following assumes a TTCAN network as the bus system, this not being with regard to the subsequent object of the present invention. Instead the object of the present invention to be explained later may also be used with other comparable bus systems such as FlexRay.
In such systems, the communication cycle is determined by a basic cycle, as already mentioned, i.e., by a timer or a first user that repeatedly transmits a reference message in at least one predeterminable time interval over the bus system, the reference message being triggered by time-trigger information when the time information reaches a time mark assigned to the trigger information. The time information in TTCAN is the cycle time, for example, which is predetermined by the local time, i.e., the local clock of the timer or the time master and by the reference message. If this time information, i.e., the cycle time of the TTCAN, reaches a certain time mark, a trigger is always triggered when this time mark is reached to start the particular reference message. The time master in the bus system therefore specifies the time for the bus system according to the basic cycle. If there should now be a shift in such a basic cycle, the TTCAN protocol, for example, offers the option of shifting the communication by setting a bit in such a reference message. Such a shift is necessary in particular to synchronize the TTCAN bus to the phase, in particular an external time base, e.g., when the event synchronized time-triggered communication option is used in the TTCAN. However, two actions are necessary for this in TTCAN according to ISO 11898-4. First, the time master or host must set a bit, known as the Next_is_Gap bit, in the reference message and then wait until the corresponding time interval, the time gap in which the communication is stopped, has begun to then restart the next reference message during synchronization by activating the time trigger. However, the time master or external time base, in particular a host controller, may not perform such a shift or synchronization with a single access; instead, it is necessary to wait for the return messages, i.e., the reference message having the Next_is_Gap bit, and the start of the time gap. Furthermore, a basic cycle may only be lengthened with this method but may not be shortened.